Photocatalytic Synthesis of Azetidines and Cyclobutanes via Energy Transfer-Mediated [2 + 2] Cycloaddition

We report a [2 + 2] cycloaddition reaction enabled by triplet energy transfer (EnT) for the efficient construction of four-membered frameworks, including azetidines and cyclobutanes. The reaction proceeds under mild conditions with alkenes bearing biologically relevant motifs, such as 6-azauracil and coumarin, delivering products with excellent diastereo- and regioselectivity. The protocol features operational simplicity, requires no external additives, and affords the desired products in high yields with minimal side reactions.